The present invention generally pertains to a process for the rapid charging of maintenance-free lead batteries with a fixed electrolyte.
The standards of manufacture which have been achieved in producing maintenance-free lead batteries (which allow for practically gas-tight, and in particular, position-independent operation) have renewed interest in many special applications and uses for the lead storage battery. Another factor contributing to increased ease of operation is the ability to charge a maintenance-free battery with relatively high currents, so that it becomes fully available for use within only a few hours. However, conventional charging processes do not adequately account for the special circumstances of cells with fixed electrolytes.
For the rapid charging of lead storage cells with a liquid electrolyte, the known two-stage charging process (according to an I/V characteristic curve) is generally sufficient. In a first stage of this process, charging proceeds with a high constant current (I) until gassing begins, at a given voltage (V). In a second stage of this process, continued charging proceeds at a constant cell voltage of about 2.4 volts, until the current developed by this constant charging voltage decreases to about 1/20th of the five-hour discharge current. Advantages of this charging method, for cells with a liquid electrolyte, are that the initial charge state is immaterial and that a fully charged cell is obtained in all cases.
At the transition from the first to the second charging stage, the charging reaction at the positive electrode EQU PbSO.sub.a4 +2H.sub.2 O.fwdarw.PbO.sub.2 +H.sub.2 SO.sub.4 +2H.sup.+ +2e.sup.- (1)
is increasingly accompanied by the secondary reaction EQU H.sub.2 O.fwdarw.1/2O.sub.2 +2H.sup.+ +2e.sup.- (2)
and the charging reaction at the negative electrode EQU PbSO.sub.4 +2H.sup.+ +2e.sup.- .fwdarw.Pb+H.sub.2 SO.sub.4 (3)
is increasingly accompanied by the secondary reaction EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 (4)
Both secondary reactions, which represent the decomposition of water, are associated with a voltage swing which progresses in the positive direction at the positive electrode, and in the negative direction at the negative electrode. As a result, the cell voltage, which is held constant during the second charging stage, is reached. Since these voltage swings indicate that the actual charging reactions (1) and (3) are being replaced by the parasitic secondary reactions (2) and (4), such voltage swings are therefore suitable as control variables for switching from constant-current charging to constant-voltage charging in a two-stage process for cells with a liquid electrolyte.
However, in the case of maintenance-free lead batteries with a fixed electrolyte, the reduction of oxygen essentially occurs as a parasitic secondary reaction at the negative electrode. The distinct voltage decrease which is seen at the negative pole of an open cell is absent. The previously described two-stage charging process has therefore not always been satisfactory in connection with such cells. Moreover, experience has shown that cell capacity tends to drop during cycled operation, and that the resulting deficit cannot be compensated for without creating an additional loss of water (and thus electrolyte).
Consequently, the manufacturers of maintenance-free batteries, in their brochures, often indicate modified charging methods which, as a rule, are effective only for a certain series of storage batteries. This is because there are often very substantial differences in design between such series, including the way in which the electrolyte is fixed, namely, by gelling or by means of highly absorbent mats.
One such modified charging method replaces conventional I/V charging with a three-stage I.sub.1 /V/I.sub.2 charging method. The special feature of this charging method is that during the voltage-controlled charging phase, which is short, the charging process does not wait until the self-regulating charging current decays. Rather, when the charging current has decreased to a defined value I.sub.2 (a certain percentage of the five-hour charging current), charging continues at this constant current (I.sub.2) until the end of the anticipated total charging time. If, for example, I.sub.2 =80% of the five-hour current, and assuming the cell was previously completely discharged, the total charging time is generally between 13 and 15 hours.
Two other charging processes are known, one which operates with a constant charging current throughout the entire process, and another which operates with a constant charging voltage throughout the entire process. The constant charging voltage process is economical, and is widely used for gas-tight alkaline batteries. However, in a sealed lead storage battery, unregulated charging can result in elevated water loss. For constant charging currents, the supply voltage must be adjusted to the characteristic charging and temperature behavior of the battery. Otherwise, overcharging (with a possible increase in water consumption) or insufficient charging would result.
These and other charging methods for gas-tight lead cells are also not always equally favorable for different applications, such as when cyclically loaded or when placed on standby while maintaining a charge.